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Question about actinic lighting

I have a question about actinic lights in a planted tank. I know actinic lights don't help the plants out, but what effect does it have on possible algae growth and total wattage?

Let me explain. I am setting up my 5th aquarium. Two existing tanks are semi-low tech - about 1wpg of low output T5 with filters. The other two tanks are Walstad setups with soil substrate and no filtration. The latest setup is a 46 bowfront I acquired and I am planning on another Walstad type setup. The light fixture I ended up with is a 2x21w low output T5, but it is a saltwater fixture so it came with one actinic bulb. I have plenty of replacement bulbs that can go in there in the 6500K-10000K range if I want to.

What I am curious about is this - with one bulb in the spectrum for plant growth, what effect will the actinic bulb have? The color of the bulb - very blue - provides some cool colors. If the actinic bulb isn't doing anything for the plants, will in encourage algae growth, or is the color temp of the bulb out of the range that even matters in freshwater? In other words, does it even count in the light intensity/wattage equation or is it just blue eye candy?

Just curious if anyone has an opinion or experience with an actinic bulb over a freshwater planted tank.

Wont even count for anything in the light usage for the tank. Just "Eye candy" Its possible it could help induce algae. BUT I think there are other things that cause algae issues long before a actinic light would. Poor water conditions or lacking specific nutrients being first on the list.

Blue actinic lighting is in the wavelength range of about 420nm-460nm. PAR, the spectrum that plants actually use is approximately 400nm-700nm with distinct peaks in activity in the mid to upper 400's and the mid to upper 600's. I would love to hear a logical explanation why actinic lighting is useless for plants. It isn't of course, but I enjoy watching folks try to explain why it is. For a real treat, try growing plants under just blue actinic and just under a red bulb in the 680 nm range and observe the results. OK, nobody here's going to get off their soapbox long enough to actually do it, but what you'll find is that plants will develop somewhat normally under the blue, although the internodal distance will be greatly decreased, essentially dwarfing the plant. It will not flower normally however. Under just the red the plants will elongate so rapidly they won't be able to support themselves, and will not develop in any way remotely resembling normal.

Algae isn't caused by light. It is caused by nutrients in the water. The connection people often make is because algae often isn't as picky and so can handle higher intensity then plants (hence why having too much light can encourage algae).

The actinic is perfectly fine for the plants and it won't cause algae.

And now I'm tempted to throw some actinic bulbs (or maybe something very high in kelvin) into the fixtures over my planted tank. I actually like the sound of dwarf plants, especially now that my tank has been taken over by tall plants that race to the surface then branch all over the place.

“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.” - Terry Pratchett

1. Actinic lighting matches the peak absorbancy spectrum of chlorophyll a. This pigment is found in all photosynthetic organisms from plants to algae. It will grow plants without a doubt. Modern hydroponic LED lights use blue-violet and Red lights to achieve best growth with minimal waste (there is no reason to put green light in a plant growth lamp).

2. Actinic lights have low measured PAR values because the PAR meter sensors (Apogee in particular) do not have a good response below 470 nm. At 420 nm only 40% of the light emitted is actually detected by the meter. If you have a lamp that has most of it's energy below 450 nm and peaks at 420 nm it will always read low even though the amount of light energy released is the same as a daylight lamp.

3. Actinic light is near the nonvisible range of the EM spectrum and looks dim to the human eye. This is probably how the "not bright enough" rumor got started. The lamp is producing plenty of light that plants can use.

Plants do need to balance blue and red light. In a 6700K or 10K lamp there is a good balance of blue and red. There is also about a third of the light in the green wavelengths that is useless except to make the tank look pretty to the eye. Although an actinic will grow plants it really needs to be balanced with a second lamp that has more red in it for the best results.